I. Field of the Invention
This invention relates generally to a portable lathe for preparing the ends of tubular or solid cylindrical workpieces of circular cross-section, and more particularly to a tool advancing mechanism for cutting or otherwise machining the workpiece as the tool is made to rotate about the workpiece.
II. Discussion of the Prior Art
The general arrangement of portable lathes for machining either the interior or exterior surfaces of a tube or pipe are known in the art. For example, my earlier U.S. Pat. No. 4,739,685 describes a split frame portable machining lathe having a clamshell structure adapted to be clamped about the tubular workpiece and which includes a rotatable member in the form of a ring gear journaled for rotation relative to an adjacent stationary member which is clamped to the workpiece in question. The ring gear is arranged to be driven by a suitable motor at a predetermined speed and affixed to it is a tool holder which can be made to move in a radial direction by means of a conventional star wheel drive. In such a star wheel drive, each time the tool holder rotates past a fixed trip pin mounted on the stationary portion of the clamshell, the star wheel is rotated one tooth pitch and, in doing so, rotates a threaded rod or lead screw to which a tool holding block is affixed Thus, upon each revolution, the tool holding block moves radially an incremental amount determined by both the tooth pitch of the star wheel and the pitch of the threaded rod driven by the star wheel.
While the above-described star wheel configuration provides a workable way of incrementally advancing a cutting tool relative to a tubular workpiece, it suffers from a number of defects, not the least of which is frequent breakage or wear of the trip pin. When it is considered that portable pipe end finishing lathes are frequently used in effecting repairs in piping used in power plants, including nuclear power plants, it is important that the device work properly over prolonged periods of time to avoid unnecessary exposure of human maintenance personnel in areas where they may be exposed to harmful radiation.
Not only is the star wheel system for advancing the cutting tool subject to breakage, but also it permits only one tool feeding rate. That is to say, once the tooth pitch and thread pitch of the star wheel assembly are set, the tool feed rate can no longer be readily varied to suit a particular application.